1. Field of the Invention
This invention relates to velocity measurement apparatus, more particularly to an apparatus for measuring the velocity of a moving target by using the Doppler shift of pulsed ultrasonic waves.
2. Description of the Prior Art
Velocity measurement apparatus using the Doppler shift of ultrasonic pulsed waves are known. For example, such an apparatus, is used for measuring the velocity of blood-flow. This type of velocity measurement apparatus basically comprises a transmitting transducer which projects ultrasonic pulsed waves to a moving target, a receiving transducer for detecting echoes from the target, a phase detector or comparator for sensing the Doppler shift due to the velocity of the target, and a device which transforms the output of the phase detector into velocity information.
In order to operate this type of apparatus effectively, the following relationships must exist: EQU .DELTA.f .ltoreq. 1/(2T) EQU d = (CT)/ 2
where .DELTA.f is the Doppler shift frequency, T is the pulse repetition pitch or interval, C is the velocity of sound in the medium in question, and d is the distance between the transducers and the target.
Since the Doppler shift frequency .DELTA.f is proportional to the velocity of the moving target, it appears that the maximum detectable velocity of the target is small if the distance involved is large. This fact poses a serious problem for uses of the apparatus for blood flow velocity measurement as it becomes virtually impossible to measure the velocity of high speed blood flow deep within the body.